Towards Highly Efficient Nitrogen Dioxide Gas Sensors in Humid and Wet Environments Using Triggerable-Polymer Metasurfaces

Abstract

We report simulations on a highly-sensitive class of metasurface-based nitrogen dioxide (NO2) gas sensors, operating in the telecom C band around the 1550 nm line and exhibiting strong variations in terms of the reflection coefficient after assimilation of NO2 molecules. The unit architecture employs a polymer-based (polyvinylidene fluoride-PVDF or polyimide-PI) motif of either half-rings, rods, or disks having selected sizes and orientations, deposited on a gold substrate. On top of this, we add a layer of hydrophyllic polymer (POEGMA) functionalized with a NO2-responsive monomer (PAPUEMA), which is able to adsorb water molecules only in the presence of NO2 molecules. In this process, the POEGMA raises its hidrophyllicity, while not triggering a phase change in the bulk material, which, in turn, modifies its electrical properties. Contrary to absorption-based gas detection and electrical signal-based sensors, which experience considerable limitations in humid or wet environments, our method stands out by simple exploitation of the basic material properties of the functionalized polymer. The results show that NO2-triggered water molecule adsorption from humid and wet environments can be used in conjunction with our metasurface architecture in order to provide a highly-sensitive response in the desired spectral window. Additionally, instead of measuring the absorption spectrum of the NO2 gas, in which humidity counts as a parasitic effect due to spectral overlap, this method allows tuning to a desired wavelength at which the water molecules are transparent, by scaling the geometry and thicknesses of the layers to respond to a desired wavelength. All these advantages make our proposed sensor architecture an extremely-viable candidate for both biological and atmospheric NO2 gas-sensing applications.

Keywords: frequency-selective surface; gas sensors; metasurfaces; optical sensing.

Figure 1

The structure and operating principles of the metasurface-based NO${}_2$ gas sensors: (a) the layers comprising the architecture with their respective thicknesses; (b) example of operating the reference and test metasurfaces: the reference cell (left side) is operated in the absence of NO${}_2$ molecules, and the PAPUEMA-POEGMA layer exhibits reference values of the relative permittivity ${ϵ}_r$ and electric conductivity $\sigma$. The test cell (right side) is operated in the presence of NO${}_2$ molecules, which increase the hydrophyllic state of the PAPUEMA-POEGMA and considerably increase both ${ϵ}_r$ and $\sigma$. The absorption readout of the cell under test is modified considerably with respect to the reference cell, usually in the form of prominent absorption peaks, if the reference is set to a fully-reflective behavior; (c) the incomplete ring metasurface layout; (d) the rod metasurface layout; and (e) the cylinder metasurface layout.

Figure 2

Results obtained for the half-ring structure: (a) layout of the ring elements and element sizes chosen for the PVDF and PI in order to have a resonant response around the working wavelength; (b) spectral behavior of the PI half-ring element structure at different humidity levels and (c) at different input polarization states; and (d) spectral behavior of the PVDF half-ring element structure at different humidity levels and (e) at different input polarization states. For subfigures (b–e), the insets show the local field enhancement in the interface plane taken at maximum resonance.

Figure 3

Results obtained for the rod-based structure. (a) Layout of the ring elements and element sizes chosen for the PVDF and PI in order to have a resonant response around the working wavelength; (b) spectral behavior of the PI rod-element structure at different humidity levels and (c) at different input polarization states; and (d) spectral behavior of the PVDF rod-element structure at different humidity levels and (e) at different input polarization states. For subfigures (b–e), the insets show the local field enhancement in the interface plane taken at the resonance peak.

Figure 4

Results obtained for the disk-element architecture. (a) Layout of the ring elements and element sizes chosen for the PVDF and PI in order to have a resonant response around the working wavelength; (b) spectral behavior of the PI disk-element structure at different humidity levels and (c) at different input polarization states; and (d) spectral behavior of the PVDF disk-element structure at different humidity levels and (e) at different input polarization states. For subfigures (b–e), the insets show the local field enhancement in the interface plane taken at the resonance peak.